Goetz P. Hellmann

Fully Reversible Shape Transition of Soft Spheres in Elastomeric Polymer Opal Films

Core-interlayer-shell (CIS) beads featuring noncross-linked hard cores were used to prepare large and well-defined elastomeric opal films with remarkably distinct iridescent reflection colors. The matrix of the opal films was cross-linked by UV-irradiation after compression molding of the CIS beads mixed with a bifunctional monomer. Stress-induced deformation of the embedded PS cores lead to hexagonally arranged spheroid oblates with an aspect ratio of 2.5. Optical characterization shows that bead deformation provokes a tremendous photonic band gap shift of about 160 nm. Fully reversible shape transition from the spheroid oblates back to the spherical beads and hence full recovery of the original photonic band gap can be achieved.

Thermo-cross-linked Elastomeric Opal Films

An efficient and convenient thermal cross-linking protocol in elastomeric opal films leading to fully reversible and stretch-tunable optical materials is reported. In this study, functional monodisperse core-shell particles were arranged in a face-centered cubic (fcc) lattice structure by a melt flow process. A problem up to now was that un-cross-linked films could not be drawn fully reversibly and hence lost their optical and mechanical performance. After thermal cross-linking reaction, the obtained films can be drawn like rubbers and the color of their Bragg reflection changes because of controlled lattice deformation, which makes the cross-linked films mechanochromic sensors. Different techniques were developed for the cross-linking of the films a posteriori, after their preparation in the melt flow process. A photo-cross-linking approach was reported earlier. This study now deals with a very efficient thermo-cross-linking approach based on the chemistry of hydroxyl- and isocyanate-functionalities that form urethane bridges. The focus of the present work is the mechanism and efficiency of this cross-linking process for elastomeric opal films with excellent mechanical and optical properties.

Electric-field-tuned color in photonic crystal elastomers

Electrically tuned photonic crystals are produced by applying fields across shear-assembled elastomeric polymer opal thin films. At increasing voltages, the polymer opal films stretch biaxially under Maxwell stress, deforming the nanostructure and producing marked color changes. This quadratic electro-optic tuning of the photonic bandgap is repeatable over many cycles, switches within 100 ms, and bridges the gap between electro-active materials and photonic crystals.

Large-scale ordering of nanoparticles using viscoelastic shear processing

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.

Paper-supported elastomeric opal films for enhanced and reversible solvatochromic response

Abstract. Paper-supported solvent-responsive elastomeric opal films based on hard-soft core-interlayer-shell spheres featuring remarkably distinct iridescent reflection colors were investigated. By using extrusion and compression molding, elastomeric opal films could be obtained, which were incorporated into a porous paper sheet to build robust composites. Swelling of the opal paper composites caused by various solvents was accompanied with a tremendous photonic band gap shift of the reflection colors. The combination of the extraordinary optical properties of the elastomeric opal films used with the remarkable features of highly porous paper can be the basis for a whole family of polymer-based soft sensors featuring a fascinating optical, fast and reversible response.

Morphology-Controlled Coating of Colloidal Particles with Silica: Influence of Particle Surface Functionalization

We present a general, convenient, and efficient synthetic concept for the coating of colloidal particles with a silica (SiO2) shell of well-defined and precisely controlled morphology and porosity. Monodisperse submicroscopic polystyrene (PS) particles were synthesized via two-stage emulsifier-free emulsion polymerization and subsequent swelling polymerization, enabling selective particle surface modification by the incorporation of ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers, which could be proven by zeta potential measurements as well as by determining the three-phase contact angle of the colloidal particles adsorbed at the air-water and n-decane-water interface. The functionalized particles could be directly coated with silica shells of variable thickness, porosity, and controlled surface roughness in a seeded sol-gel process from tetraethoxysilane (TEOS), leading to hybrid PS@silica particles with morphologies ranging from core-shell (CS) to raspberry-type architectures. The experimental results demonstrated that the silica coating could be precisely tailored by the type of surface functionalization, which strongly influences the surface properties of the colloidal particles and thus the morphology of the final silica shell. Furthermore, the PS cores could be easily removed by thermal treatment, yielding extremely uniform hollow silica particles, while maintaining their initial shell architecture. These particles are highly stable against irreversible aggregation and could be readily dried, purified, and redispersed in various solvents. Herein we show a first example of coating semiconducting CdSe/ZnS nanocrystals with smooth and spherical silica shells by applying the presented method that are expected to be suitable systems for applications as markers in biology and life science by using fluorescence microscopy methods, which are also briefly discussed.